In outboard motors, the engine is covered with an engine cover that is composed of an upper cover and a lower cover. An external air inlet and a discharge outlet are formed in the engine cover, and the inside of the engine cover (e.g., engine room) is ventilated by driving a ventilation fan that is attached to the end of a crank shaft.
Since the engine cover is formed by injection-molding synthetic resin, in the case where the external air inlet, the discharge outlet, a duct for ventilation, etc. are formed in the upper cover, bosses and ribs need to be formed in the upper cover. In this case, a molded upper cover is complex in shape, as a result of which shrinkage cavities are prone to occur due to thermal contraction during molding. This is particularly problematic in the case of the upper cover because it is a component that influences the appearance of the outboard motor. In view of this problem, an outboard motor has been proposed in which an external air inlet and a discharge outlet are formed in a lower cover that is less influential to the appearance than an upper cover (refer to JP-A-7-71223, for example).